Modifying the transient response of image-reproducers



May 1,8, 1954 B. D. LouGHLlN MODIFYING. THE TRANSIENT RESPONSE OF IMAGE-REPRODUCERS med- Aug. 14, 195o s sheets-sheet 1- B. D. LOUGHLlN May 18, 1954 MODIFYING THE TRANSIENT RESPONSE oF IMAGE-REPRODUCERS Filed Aug. 14, 195o 3 Sheets-Sheet 2 .AIIIQEE wenn() J0 Plat-I INVENTOR. I BERNARD D. LOUGHLIN ATTOR N EY IIwEE.

May 18, 1954 B. n. LouGHLlN MODIFYING `TEE TRANSIENT REsEoNsE oE IMAGE-REPRoDucERs Filed Aug. 14, 1950y :s Sheets-sheet s ATTO R NEY Patented May 18, `1954 MODlIFYING THE TRANSIENT RESPONSE OF IMAGE-REPRODUCERS Bernard D. Loughlin, Lynbrook, N. Y., assignor to Hazeltine Research, Inc., Chicago, Ill., a corporation of Illinois Application August 14, 1950, Serial No. 179,122

8 Claims.

General The present invention relates: to apparatus for modifying the apparent response of image-reproducing devices to the unidirectional transients of an applied electrical signal. The term unidirectiona-l transient, as employed in the specification and claims, is intended to denote a sudden change in the amplitude of an electrical signal, such as may occur in the video-frequency portion of a detected television signal between two amplitude levels thereof coresponding to two adjacent brightness levels in the transmitted image, for example, from black to White. Although apparatus of the type under consideration is oi general application, it has particular utility in television receivers for improving the sharpness ci the images produced by the imagereproducing device thereof and, hence, will be described in that environment.

There presently exists the belief that much of the eye fatigue and other unpleasant reactions often experienced by individuals viewing the image or picture produced by a television receiver is due to.y insuiicient sharpness of the picture rather than to inadequate detail or resolution. By sharpness of the reproduced picture is meant the rate of change of brightness thereof with space, that is, sharpness is related to the appearance of a distinct edge between two areas of different brightness. The lack of sharpness of the picture being viewed is believed to cause the eye muscles of an observer to attempt to produce a sharper or better focused image on the retina of the eye. When the picture being viewed has insuiiicient sharpness, there may result a continued subconscious action on the part of the observer to focus his eyes in the manner just mentioned, thus causing eye fatigue. On the other hand, inadequate detail in the viewed pic*- ture is believed merely to iniiuence the recognition of ne detailed objects. It has been recognized that the above-mentioned distress experiencesby one observing television entertainment may be further aggravated as the use of television image-reproducing devices with larger display areas becomes more widespread. This is because one observing a larger television pioture is more likely to sit at such a viewing distance that the picture will subtend a larger viewing angle at the eye, thus making the grain or spot structure of the picture more visible.

Since the present trend in television receivers is toward the use of larger image-reproducing devices, the problem of imparting greater sharpness to the television pictures is assuming greater importance.

Considering now a complete television system including both the transmitter and the receiver, there are one or more factors which are instrumental in causing the bandwidth of a television signal translated by the system to be limited. Bandwidth limitation occurs in the scanning spot of the camera pickup tube at the transmitter, in the signal-translating channels of both the transmitter and the receiver, and in the scanning spot of the image-reproducing device of the receiver. The limitation on bandwidth imposes a corresponding limitation on the detail or resolution of the picture produced by the receiver of the television system. In conventional television receivers, the relatively limited bandwidth of the signal-translating channels thereof also undesirably places a limitation on the sharpness of the reproduced picture. Heretofore it has been the opinion of workers in the television art that the resulting limitation placed on the sharpness of the reproduced picture was a fundamental one, that is, that the limited bandwidth, imposed by the electrical circuits and the scanning spots inherently prevented the translation of the highest frequency components of an applied television signal, thus prolonging the time required for a transient to pass from one amplitude level to another, and that this necessarily resulted in a degraded transient having a gradual slope rather than a steep one, thus causing a general lack of sharpness in the reproduced picture. It has been demonstrated, however, that the sharpness of the reproduced television picture need not be limited by the bandwidth of the television signal-translating circuits and the scanning spots mentioned above but that the limited bandwidth ci' the signaltranslating stages of television apparatus, such as a television receiver, is` effective to limit only the permissible minimum` separation between two successive unidirectional transients but need not limit the permissible steepness oi a transient.

Although it is desirable, in television receivers having relatively wide pass-band characteristics of the type usually found in such receivers today, to provide apparatus in accordance with the present invention for modifying the apparent response of cathode-ray image-reproducing devices of the receivers` to television signals including unidirectional transients to increase the sharpness of the reproduced images, it may also be desirable in many applications, such as in Ylow-cost receivers having relatively inexpensive circuits characterized by their narrow pass-band characteristics, to utilize apparatus of the type under consideration to enable the receivers to produce pictures having a sharpness comparable to that of the pictures produced. by conventional television receivers.

It has been proposed in a prior television broadcasting system that, in order to reduce the pass band of the television channel and the television apparatus of the system, the scanning velocity of the electron beam ofthepickupgdevice at the transmitter be varied from its, normal velocity in accordance with the folded 'first derivative of a transient derivedlby4 the scanning operation. This folded derivative corresponds to the absolute value of the differential quotient of the transient. More particularly, the speed of the scanning beam at about the time of a transient is reduced momentarily from its nor- 'mal value and thereafter is returned to that Avalue.

. duced) from its normal value, the length of time required fory scanning -av line of predetermined length will vary depending upon the picture content. Because itis necessary to maintain the rmovement of the exploring electron beam. at

the transmitter-and thev reconstructing electron beam at theV receiver iin synchronism, synchronizing information must be transmitted for use at the receiver tocorrect possibledeviations from synchronism after each 'line scansion.

A television 'receiver constructed toy operate in themanner just described is not suitable for use with v present-day television transmitters, wherein the scanningvelocity of the electron beam ofthe pickup-device is-maintained at a predetermined substantially constant velocity. When-a receiver of `the type described is supplied with=a television signal of the type received from a television transmitter operating on. current broadcastingstandards, undesirable largearea distortion occurs A in i -the picture reconstructed-.by the receiver. .-Thisdistortion canv only be avoided by operating thedescribed receiver in conjunction witha transmitter which decreases the scanningvelocity of its pickup device in accordance with the absolute value of the differential quotientof a transient. Brieiiy,a receiver of the type described above is not compatible with television transmitters presently in use and is, therefore, generally unacceptable.

It is Y an object of the invention, ,-theiefore. to provide new and Vimproved apparatus for modifying the apparent response of an imagereproducing -device to unidirectional transients of an electrical signal which-avoids one or more of the `above-mentioned-disadvantages of yprior such apparatus.

It is another object ofthe invention to provide a relatively simple and inexpensive apparatus for use in conjunction witha television receiver for modifying the apparent response of .the image-reproducing device ofthe receiver to unidirectional transients in the television signal in such a way as to produce a picture of improved quality.

'It is a further object of the invention tov provide new and improved apparatus for use in a television receiver for modifying 'the' apparent response of the image-reproducingdevice, of the receiver to unidirectional transients inthe teleof a derivative. of an applied signal is a. signal related to a chang- `transient between two amplitude levels. the broad term derivative as used in thev specivision signal in a manner to reduce the eye fatigue and unpleasant reactions ordinarily experienced by some observers of television programs.

It is an additional object of the invention to provide an apparatus of the type under consideration for use with a `television receiver having a relatively narrow rband-pass characteristic to enable the receiver to develop a television image having a quality or sharpness comparableto that of the image produced by a conventional receiver having a much wider band-pass characteristic.

In an image-reproducing system including an lin'iage-reproducing device for reproducing an imagerepresenting an electrical signal which mayinclude unidirectional transients, an apparatus in accordance with a particular form of .the invention for modifying the apparent response of the image-reproducing device to such transients comprisesr acircuit for applying the electrical sign-al to an image-reproducing device and means for normally scanning the imagereproducing device` at a predetermined velocity. rlhe apparatus further includes control circuit means coupled to thel aforesaid circuit and including means for deriving signals representative of the iirst derivative and-of at least a higher order derivative of said transients and including a modulator device responsive to saidderived signals and effective to developua'control eifect representative jointly'ofsaid-rst derivative-and said higher order derivative of the transients of an applied signal. 4The apparatus further includes electrical means coupled tothe aforesaid control circuit means for utilizing such control effect to vary thevelocity of scanning from the aforesaid predetermined velocity successively in opposite senses within a short interval after the initiation of a transient, whereby the imagereproducing device is conditioned to produce an image corresponding to the applied signal with modified transients.

Theterm derivative 4as employed throughout the specification and claims, is intended to denote any signal having lthe general wave form Broadly speaking a derivative ingrcharacteristic Vof that applied signal, that is,

a signal which has y,a zero value when that applied Ysignal is constant in magnitude at any particular amplitude level and has some magnitude other than, zero when the applied signal has a Thus,

fication and claims, is not necessarily a simple first, second, or third derivative but may include ,nonlinearfunctions of simple derivatives, cross products between simple derivatives, time difference signals, or any other functions producing signals of the general form described above. For example, the` output signal ofa band-pass filterwhich has a video-frequency signal applied to Vits input circuit has the general wave form describedV above. Thus, in this broad sense, a-band-pass lter would comprise a network which is effective to develop a-derivative signal.

:For a better understanding ofthe present invention, together with other and further objects thereof, reference is had to the following description taken in connection with thev accompanying drawings, and its scepewill be pointed out in the appended claims.

In the accompanying drawings, Fig. 1 is a sche- 75.;m atic circuit-diagram of a completetelevision receiver including an apparatus in accordance with the prior art for modifying the apparent response of the image-reproducing device of the receiver to unidirectional transients of the television signal; Fig. la is a modied form of imagereproducing device which may be employed with the receiver of Fig. l; Figs. 2 and 3 are graphs utilized in explaining the operation of the response-modifying apparatus of the Fig. 1 receiver; Fig. 4 is a circuit diagram, partly schematic, of a response-modifying apparatus in accordance with the invention; Figs. 5 and 6 are graphs employed in explaining the operation of the apparatus of Fig. 4; Fig. '7 represents another response-modifying apparatus, and Fig. 8 is a graph utilized in explaining the operation of the apparatus represented in Fig. 7.

General description of Fig. 1 receiver Referring now more particularly to Fig. 1 of the drawings, the television receiver there represented comprises a receiver of the superheterodyne type including an antenna system I 0, It coupled to a radio-frequency amplifier I2 of one or more stages. There is coupled to the latter unit in cascade, in the order named, an oscillatormodulator I3, an intermediate-frequency ampliiler I-lr of one or more stages, a detector and automatic-gain-control or A. G. C. supply unit I5 having a pair of output terminals 30, 30, a response-modifying apparatus 24 preferably including a conventional video-frequency amplier I6 of one or more stages, and a cathode-ray image-reproducing device I'I of conventional construction. The brilliancy-control electrode of the image-reproducing device Il is connected to the high-potential output terminal 3! of unit I6. Unit I'I also includes the usual line-frequency and held-frequency scanning coils I8 and I9, respectively, for deiiecting the cathode-ray beam in two directions normal to each other. The A. G. C. supply circuit of unit I5 is connected to the input circuits of one or more of the stages of units I2, I3 and I 4 by a control circuit conductor 56. Connected to the two output terminals of the intermediate-frequency amplifier t4 is a conventional sound-signal translating apparatus 25 and a sound reproducer 26. An output circuit of the video-frequency amplifier IG is coupled to the input circuit of a linefrequency generator Z2 and a field-frequency generator 23 through a synchronizing-signal amplifier and separator 2d and an intersynchronizingsignal separator 2i. The output circuits of the generators 22 and 23 are coupled in a conventional manner to the scanning coils I8 and I 9, respectively, of the image-reproducing device I'I. The units Iii-26, inclusive, may be of conventional construction and operation so that a detailed description and explanation of the operation thereof are unnecessary herein.

General operation of Fig, 1 receiver are derived by the detector I5 and are supplied to the video-frequency amplifier I6 wherein they are amplified and from which they are supplied to the input circuit of the image-reproducing device il'. The operation of the unit 24 with reference to unidirectional transients Will be explained in detail hereinafter. A control voltage derived by the automatic-gain-control supply I5 is applied as an automatic-amplifcation-control bias to the gain-control circuits of units I2, I3 and lli to maintain the signal input to the detector of unit I5 within a relatively narrow range over a wide range of received signal intensities.

Unit 26 selects the synchronizing signals from the other modulation components of the composite television signal applied thereto from the video-frequency amplier I6. Line-synchronizing and field-synchronizing signals derived by the separator E@ are separated yfrom each other by the unit 2| and are then supplied to individual ones of the generators 22 and 23 to synchronize the operation thereof. An electron beam is produced by the cathode-ray image-reproducing device I'I and the intensity of this beam is controlled in accordance with the video-frequency and control voltages impressed on the Iorilliancy-control electrode by the input circuit including the terminal 3| of the image-reproducing device. Sawtooth waves are generated in the line-frequency and the field-frequency generators 22 and 23, respectively, and are applied to the scanning coils it and E19 to produce scanning fields thereby to deflect the cathode-ray beam of the image-reproducing device il in two directions normal to each other to trace a rectilinear scanning pattern on the screen of the device and thereby reconstruct the translated picture.

rl"he audio-frequency modulation components of the received signal are derived in a well-known manner by the sound-signal translating apparatus 25 and are applied to the sound reproducer 25 wherein they are converted to sound.

Description of response-modifying apparatus of Fig. 1

Referring now more particularly to the unit 213, that apparatus for modifying the apparent response of the image-reproducing device I'I to unidirectional transients of the television signal applied thereto comprises a circuit for applying an input signal to that image-reproducing device. This circuit includes the connections to the brilliancy-control electrode and the cathode of the device i7 and may comprise the video-frequency amplifier I3 and any one or more of the preceding signal-translating units lZ-'I 6, inclusive, the output circuit of amplier I6 being coupled in a conventional manner to the input circuit of unit I1. This input circuit includes the terminal 3l coupled to the control electrode of unit I7 and also includes a connection comprising an adjustable tap coupled between the cathode of device il and a positive potential point on a potentialsupply means represented as a battery 36 for that device. The video-frequency amplifier I6 and the ampliiier stages of units lel, inclusive, may have a frequency pass band corresponding to that of conventional such ampliersior example, one having a sharp cutoff characteristic at the highfrequency end of vits pass band. Alternatively, the over-all frequency pass band of the receiver may be considerably less Ithan that of 'a conventional television receiver so that the translation of unidirectional transients by the channel tends to be impaired due to its inability to translate the very high-frequency components.

The .apparatus 24 for modifying the apparent response off'the image-reproducing device ll to unidirectional transients `also includes electrical means Ifor normally scanning the device I'i at a predetermined` velocity. Thev terni electrical means, as employed in the specification and claims, is meant to include such an element or elementsas an electrostatic plate or plates cr an electromagnetic Winding or windings which are suitable for controlling or influencing the deflection of an electron beam. In particular, this electrical means comprises electromagnetic means inthe form of the described line-scanning and field-scanning coils i8 and i9, respectively. rhe response-modifying apparatus in accordance with the invention further includes control circuit means coupled to the circuit for applying the video-frequency signals to the image-reproducing device l1, the aforesaid means being effective to develop a signal representative of a derivative lof the transient of an applied signal. This control circuit means comprises a series-coupled ccmbination including a differentiating circuit 2i of conventional construction and video-frequency amplifier 28 which is coupled to the output circuit lof the amplier V5 for developing a first derivative of a unidirectional transient appearing in. the video-frequency signal. For some applications, it may be desirable to couple the differentiating circuit 21 to the input circuit of the videofrequency amplifier l5, as represented by the conductor 29 sho-wn in broken-line construction, rather than to the output circuit thereof in order that the output signal of unit 2E shall have the same time delay as the signal applied to the brilliancy-control electrode of the image-reproducing device l1.

The response-modifying apparatus 2e additionally includes electrical means for utilizing the control effect or first derivative of the unidirectional transient developed by the units 2'! and 28 to vary the velocity of scanning from the aforesaid predetermined velocity successively in opposite senses within a short interval after the initiation of a transient, whereby the image-reproducing device il is conditioned to produce an image corresponding to the applied video-frequency signal with modified transients. This means may lbe of the same type as the scanning mean-s including the coils i8 and i9, that is, of the electromagnetic type, or may be of a 4different type as will be mentioned subsequently. According to one form of the invention, therefore, this means comprises an auxiliary winding 33 which may, for example, be disposed adjacent to the line-scanning coil i8, for exam-ple between the conventional focus coil (not shown) and the aforesaid line-scanning coil. rlhis winding 33 is so oriented that it is effective to produce a deflection in the same direction as that produced by the line-scanning coil le. One terminal of the Winding 33 is grounded and the other terminal 32 thereof is coupled to the high-potential output terminal of the amplifier 28.

Operation of response-modifying apparatus of Fig. 1

The operation of the response-modifying apparatus 24 represented in Fig. 1 and the results obtained thereby may be best understood With reference to thc curves of Figs. 2 and 3. Referring first to Fig. 2, curve A thereof represents to a veryY enlarged scale a fragmentary portion of an amplified video-frequency signal supplied in a single line scansion to terminal 3|. The

pulseisignal of curve Amay represent a vertical White bar in the picture, the edges of the bar having been degraded, as indicated by. the rounded. corners and sloping edges of thepulse, by the limited pass band of some preceding portion of lthe television signal-translating channel. This signal is an amplified form of the signal supplied by the output terminals 30, 30 of the detector l5 to the response-modifying apparatus 26.' At time to a positive-going unidirectional transient of the television signal is initiated and terminates at time t2. Following the transient the unidirectional signal continues at a substantially uniform amplitude level until time t3 whereupon a second and negative-going unidirectional transient occurs and terminates at approximately time t5. For the purpose of the explanation which follows, it will be assumed that the band-pass characteristic of the transient response-modifying apparatus 24 of Fig- 1 including, the amplifier I6, when unmodified by the action of units 21, 28 and 33, is incapable of faithfully translating transients steeper than those represented in curve A of Fig. 2.

The differentiating circuit 21 responds to the transients described in the preceding paragraph and, by the well-known differentiating operation, develops an output signal, which after amplication in unit 2S, is represented by curve B of Fig. 2.

rThe output signal of units 21 and 28 comprises a control effect which is representative of the first derivative signal of that of curve A. In the absence of the application of the first derivative signal represented by curve B to the auxiliary winding 33, the variation of the line-scanning current or field with time is substantially linear during the entire interval tri-t5, as shown by the sloping line C which includes the broken-line portions 4immediately beneath curve B of Fig. 2. As previously stated, the Winding 33 produces deflection in the same direction as the line-scanning coil IS. Consequently, the application by the units 2l and 2B of the first derivative signal to the Winding 33 modies the scanning field developed by the complete scanning system, comprising the Winding 33, i3 and I9 of the imagereproducing device Il, during the intervals tui-t2 and 25a-t5 so that the effective scanning current or field varies With time in the manner represented by the solid-line curve C.

It will be seen from curve C that the resultant scanning field departs from linearity by increasing in magnitude during the interval tri-t2 and then proceeds to vary linearly during the succeedi-ng interval lf2-t3. During the next succeeding interval 7f3-t5 the resultant scanning field again departs from linearity by decreasing in amplitude. After the time t5 the rate of variation is again normal, or linear. Since the velocity of scanning by the electron beam of the image-reproducing device Il is related to the first derivative of the field represented by curve C, the scanning velocity varies from its predetermined or normal constant value during the intervals to-tz audits-ts as represented by curve D. Thus the application of the first derivative signal of the unidirectional transients represented by curve A of Fig. 2 to the Winding 33 causes the velocity of scanning to have a variation as represented by curve D. During the first portion tn-ti of the intervento-t2, the scanning velocity increases from its normal or predetermined value and then during the second portion ti-tz of that interval it decreases from .the -aforesaid `predetermined value andreturns atapproximately time t2 to its no1'- mal value. During the iirst portion tsl-t4 of the next transient interval ts-tt, the scanning velocity is reduced with reference to its predetermined value, and then, during the second portion tri-ts thereof, the scanning velocity increases with respect to the predetermined velocity. Thus it will be seen from the shape of the scanning velocity curve, curve D during each of the transient intervals to-tz and ifa-t5, that the units 21 and 28 by their action on the winding 33 are effective to vary the Velocity of scanning of the imagereproducing device ll from a predetermined velocity successively in opposite senses Within a short interval after the initiation of transients.

The correct polarity of the first derivative Which is applied by the units 2'! and 28 to the auxiliary scanning Winding 33 of the image-reproducing device i is that polarity Which initially accelerates the scanning during a black-towhite transition in the video-frequency signal and which initially decelerates the scanning during a white-to-black transition. It Will be shown subsequently that this has the eiect of narrowing the White' areas of the image reproduced by unit I'! and widening the black areas thereof.

The desired result produced by the described modication of the scanning eld to produce a resultant scanning eld of the type represented by curve C of Fig. 2 may best be understood by referring to Fig. 3 of the drawings. In Fig. 3, the solid-line curve A represents a fragmentary portion of the video-frequency signal during the interval ta-td and corresponds to curve A of Fig. 2. This curve A may also be considered to represent the instantaneous beam current-space curve as well as the brightness-space curve of the image-reproducing device il when the scanning thereof has a constant velocity and the device i1 has a linear control-signal brightness characteristic. The dash-line curve E with its solid-line connecting portion during the interval tb-tc represents the instantaneous beam currentspace curve of the image-reproducing device il, ignoring velocity effects, when the transientmodifying apparatus is in operation so that the resultant scanning eld is as represented by curve C oi Fig. 2. The dash-dot-line curve F with its solid-line connecting portion during the interval trl-te represents the brightness-space curve of the image-reproducing device l1 ignoring velocity effects, when the transient-modifying apparatus is in operation. Curve F differs from curve E because, for a given instantaneous beam current, the brightness is inversely proportional to the instantaneous scaning velocity. Thus the brightness-space curve F increases gradually during the first portion or" the interval ta-tb because the velocity of scanning of the image-reproducing device is greater during that portion of the interval. However, during the second portion of the interval ifa-tb the scanning velocity decreases in a manner represented by curve D of Fig. 2. Accordingly, the brightness increases suddenly, thus producing a sudden change in the brightness-space characteristic. However, this brightness may increase to such an extent that an overshcot occurs in curve F due to the cathode-ray beam momentarily coming to rest and then moving on again over the screen of the image-reproducing device.

During the interval tig-tc, the brightness-space curve follows the solid-line curve A. However, at approximately time te the scanning velocity decreases in a manner similar to that represented in the corresponding portion of curve D of Fig. 2. Consequently another overshoot representing high 1crightness may occur in curve F during the first portion cf the interval 'tc-td. During the second portion of the interval han, the brightness curve F decreases abruptly since the scanning velocity is now greater than normal that portion of the interval.

Thus it will be seen from the curves of Fig. 3, particularly curves A and F thereof, that the derivative signal applied to the Winding 33 so iniiuences the complete scanning system of the image-reproducing device Il that there are produced transients in the reproduced image which are much steeper than the transients in the applied video signal. Hence, the image-reproducing system including the image-reproducing device Il and the unit 2d is effective to produce an iinage corresponding to the video-frequency signal applied to the output terminals 3G, 3i? of unit le, but with mcdiied or steeper transients. It will be apparent from curve F of Fig. 3 that the transient-modifying system is eiective to cause the white elements vof the reproduced picture to be narrower; hence the darker elements thereof are correspondingly Wider. Also, the overshoot which may occur in the vicinity of a transient tends tc produce some highlighting of the reproduced image. These phenomena indicate that small geometric distortions are present in the reproduced picture. These small distortions, rather than being objectionable, have been found to be desirable in that they enhance the apparent contrast of the picture and tend to compensate for the finite size of the scanning spot of the image-reproducing device.

if the complete signal-translating channel of a television receiver has a pass band of 4 megacycles with a sharp cutoff characteristic, undersirable oscillatory components may be produced in the channel by a steep transient in the applied signal. 'Ihe transient-modifying system described above may undesirably accentuate these oscillatory components incidental to modifying the transient response. In a television image-reproducing system of the type under consideration, the production of such undesired oscillatory components may be minimized by limiting the pass band of the complete signaltranslating channel or giving it a gradual cutci characteristic, or beth. This restricted pass band and gradual cutoff characteristic of the complete signal-translating channel produces some degradation of the slopes ci steep transients. However, the transient-modifying apparatus more than compensates therefor and is eiective to reproduce an image with sharper transients than those of the applied signal without the effects of annoying oscillatory components.

Considered from a somewhat different standpoint, the video-frequency ampliiier It may be regarded as at least a portion of the main signaltranslating or intensity-modulation channel for applying a television signal to the brilliancycontrol electrode of the image-reproducing devvice il, while the units 2l, 28 and 33, when the input circuit of unit 2"! is coupled to the output circuit of unit i5 by the conductor 29, may be considered as an auxiliary velocity-modulation signal-translating channel for eiectively applying the very high frequency components of the video-frequency signal to the device i 1. This viewpoint is particularly significant when the amplifier stages of units 245, inclusive, have a Wide pass-band characteristic and the videovvhich may'have a gradual: cutoff characteristic.

InV the case just mentioned, the amplier i6 is' unable to translate to the device i1 the very high frequency components comprising the steep transients of the television signal. These components are, however, eectively applied 'to the device il by units 2T, 28 and 33. In fact, when the brilliancy-control electrode of the image-reproducing device I'i of a television receiver of the typerepresented in Fig. 1 is disconnected from the amplier l 6 and the adjustable tap coupled to the cathode of device i1 is adjusted so that there is a visible"raster, a signal related to the highfrequency' components of the television Vsignal may be observed on the screen of the device due solely to scanning velocity modulation of the type described above.

The following relations, which are well known.

to those skilled in the art, also confirm the preceding statement, namely: the brightness of the image produced by the device'i is inversely proportional to the beam-scanning velocity; the

scanning velocity for the system under consideration is a constant plus the second derivative of a transient in the video-'frequency signal; and the second derivative of that transient is related to the high-frequency4 components ofthe videofrequency signals.

When the very high'frequency components of a signal are not translated by the video-'frequency amplieri but are Ueffectively applied to Ythe However, thehigh-frequency components of the' impulse' are electively applied to *the imagereproducing device by scanning velocity modulation so""th`at'the reproduced' impulse appears to have a large peak brightness-"and anarrowwidth l of the order of that 'which would be obtained with a'video-requ'ency amplifierhaving awide pass band.l This large peak'brightness is obtained lwithless instantaneous beam current b`y`- slowing down the scanning spot.- The smaller" value of the instantaneous beam current at the peak'of the impulse results in a smaller spot size.'A As a result of the smaller spot size, an additionalf improvement is obtained in' the sharpness of'the" Also better resolution-can reproduced picture. be obtained from the image-reproducing device at a higher peak brightness when the "iii'ie Adetail isapplicd by scanning velocity modulation-'as described.'

Description of Fig. la moinlificattion` Y Referring now to Fig. la of the drawings, there is represented an image-reproducing device I7 including a somewhat different element which forms a portion of a response-modifying apparatus. The device Il and its associated circuits correspond with but one exception to those represented in Fig. 1. The image-reproducing device is adapted to be connected to the intensity-modulation terminal 3l, to the velocity-modulation terminal 32,' and to line-frequency and field-- frequency generators'in the manner represented in connection with the image-reproducing device includes'within "the" envelope .thereof a fsuitable beam-deecting: electrode 29 'to which the `i-lrst i derivativefof *a transientis lapplied to varyr'the velocityofi'scanningzof 'the device I'l in the general":mannertpreviously `explained inY connection with ftheecorresponding. device of the Fig. l rei ceiver;` It may "be "preferable for some applications-.topositionl thedeflecti'ngelectrode 29 :near the#signal-input@electrodes of the electron-gun.'

structure f of ithef image-reproducing deviceY I l This frnay befaccomplished without' causing any particular `defocusingof` the spot-produced on thefscr'een offthefdevice I1 lsince the amplitude ofc' the signal'irequired for application to` the delecting.- electrode Vin thefpositioni'just men- Because the tioned :wouldfbe extremely small. deiiection sensitivityof a deiiecting electrodey positi'oned :nearfthe signal-input electrodes of the electronegunf'structure'fis highgiit is possible to includeia simple'differentiatingcircuit withinv the` envelope? f of the i Aimage-reproducing device I1 #tof-obtain :the: desired sharpening i eiects` of thertype :previously 'fconsideredv It will be clear from the foregoingfdescriptionv of fFige'l; that =the windings I8 and vIE: andthe de'ectingelectrode f'effectively comprise-scanning-means .offdifierent'types for controlling the electron'fbearnof'the device Il.' It will also be clearthat 4`deflecting plates may also be employed in' place or the `windings l iand l 9 in conjunction with the lde'ilecting electrode 29 or inA conjunction with *an* auxiliary 1 windingfsuch as the winding 33'represented in Fig: l.

Description offF'ig. 4 'apparatus Itrhas "been explained in connection. with the curves 'of Figs.: 2 Iand* 3,- inr particular with `refervaries-the excitation of-the `scanning system of theimage-'reproducing device-'l1 in a manner relatedto the second 4derivative of the unidirectional 'transient in the video-frequency amplifier applied'to the unit I6. Such aresponse-modiiy'- ing apparatus is'represented in Fig.' 4.

Referring nowto Fig. 4 of the'drawings, the apparatus in accordance with the invention there represented is adapted to be coupled to the' tei'- minals '33, 3l) and v3i and 32 of the television receiver of Fig. l. Units corresponding to those of the response-modifying apparatus of Fig. l are designated in Fig. 4 with the same reference characters. The video-frequency amplifier H5 may have anyone of the pass-band characteristics mentioned in connection with the corresponding amplier of the Fig. 1 circuit arrangenient. Coupled between theY differentiating circuit 2'! and the amplifier 28 is a second diierentiating circuit A!) and'a conventional balanced modulatorv 4E. Also coupled between the output circuit of the first dierentiating circuit 2 and a second'input circuit of the balanced modulator 4l is ai means having a nonlinear signal-translating 13 characteristic for symmetrically limiting the first derivative signal from unit 21. This means comprises a pair of rectifier devices 42 and 43 which are connected in cascade and in opposite polarity between an output terminal of the unit 2l and a contact 45 of a two-position switch 46. The switch 4t is, in turn, connected to an input terminal of the modulator 4i. A conductive connection 4l also appears between the terminal just mentioned of unit 2l and another contact 48 of switch 4S. Resistors 4S and 5U, respectively, are connected between the corresponding terminals of devices 42 and 43 and the other output terminal of unit 2l. Connected between the junction of the devices 42 and 43 and the last-mentioned terminal of unit 2'! is a battery 52 and a resistor 53 which are eiective normally to maintain the rectifier devices in a conductive state. A Conductive connection represented by the broken line 55 may be employed between the output circuit of unit 2 and the input circuit of the amplifier 28.

Operation of Fig. 4 apparatus The operation of the response-modifying apparatus of Fig. 4 may be best explained in connection with the curves of Figs. 5 and 6. It will be seen from an examination of these figures that, with the exception of curves G and H, the curves correspond generally to those of Figs. 2 and 3. Accordingly, similar curves in Figs. 5 and 6 are identied by the same reference characters primed. Curve A represents a fragmentary portion of the video-frequency signal applied to the terminals 30, 3l? of Fig. 4 for translation by the amplier i6 to the terminal 3l. Curve B represents the output signal of the iirst differentiating circuit 2l that is a-pplied to the second dientiating circuit 4t which, in turn, derives the signal of curve G which includes the second derivative of the transient occurring during the intervals ifo-t2 and 'a-ts. Assuming initially that the switch 4B is in the position represented in Fig. 4 and that the connection 55 is omitted, the connection 47 applies the first derivative signal of curve B through the switch it to an input circuit of the balanced modulator 4i. Unit dt applies the second derivative signal of curve G of Fig. 5 to a second input circuit of the modulator. In the well-known manner, the balanced modulator 4i is responsive jointly to the second derivative signal and to the iirst derivative signal to produce a control ei'ect or signal which is effectively the product oi the two input signals. This product signal is represented by curve H of Fig. 5 and is applied to unit 2B `wherein it is amplified and then appears as a signal of the same wave form for application to the terminal 32 associated with the image-reproducing device. It will be seen that the product signal includes pulse portions having steeper leading and trailing edges than those represented for the second derivative of curve G. Also, the polarity of the product signal is opposite from the second derivative signal during each of the intervals ifs-t4 and t4-t5. The resultant scanning Wave of the image-reproducing device coupled to the terminals Si and 32 is so modied by the control effect represented by curve H that it varies from linearity during the transient intervals tn-tz and iis-t5 in the manner represented by curve C. During approximately the interval to-ti the scanning wave varies in a sense opposite to the. variation during the interval ifi-t2. A

similar variation takes place in the resultant scanning wave during the intervals ifs-t4 and t4-t5. The instantaneous beam current-space curve,

ignoring velocity effects, for the image-reproducing device associated with the response-modifying apparatus of Fig. 4 appears as represented by curve E' of Fig. 6 and the brightness space curve therefor, ignoring velocity effects, is as represented by curve F. It will be seen that the application of the product of the first derivative and the second derivative of a unidirectional transient produces a brightness-space curve having the shape of curve F. During' a first portion of the transient interval tti-tb, the brightness of the reproduced image increases slowly, then rises abruptly during a second portion of the transient interval to a relatively high value, and thereafter during a third portieri of the transient interval decreases quite suddenly whereupon, at approximately time tb, it returns to its normal value. It will be seen from curve F that the brightnessspace curve has a small amount of overshoot and a small amount of undershoot during the interval tai-tb. During the interval tc-td of the negative-going transient, the brightness-space curve of the image-reproducing device associated with the response-modifying apparatus of Fig. 4 is the mirror image of that occurring during the interval iii-tb of the positive-going transient. During the first portion or the interval tc-td the brightness undergoes a small amount of brightness undershoot followed by an interval of brightness overshoot occurring prior to time td. From a comparison of curve F of Fig. 6 with the corresponding curve F of Fig. 3, it Will be seen that the former desirably has overshoot of lower amplitude and a shape which indicates a very small tendency to cause the white areas of the reproduced image to be increased and the black areas to be decreased, thus causing a very small geometric distortion of the reproduced image which is of opposite type to the geometric distortion produced when only the first derivative is used.

When the switch d6 is connected to the contact 45, the rectifier devices 42 and i3 symmetrically clip the higher amplitude portions of both the positive and negative peaks of the first derivative signal of curve B' of Fig. 5 and apply the symmetrically limited rst derivative to the balanced modulator 4i. The operation of the response-modifying apparatus is then generally similar to that described above with the switch 46 connected to the contact 48. Referring for the moment, however, to the control effect represented by curve H of Fig. 5, this eiect is a function of the product of the two signals of curves B and G. The control eect of curve H is critical with relation to the amplitude of the transient applied to the transient-modifying apparatus. By using' a limiter system including the rectifier devices 42 and 43, the eiiects of amplitude variations of the rst derivative signal can be so reduced that the first derivative signal is effective primarily to invert that portion of the signal of curve G occurring during the interval 26a-t5 without aiecting the amplitude thereof, thereby producing a resultant signal having a wave form similar to that of curve H.

When the connection 55 is made between the output circuit of the differentiating circuit 21 and the input circuit of the amplier 28 as represented by the broken line in Fig. 4, the rst derivative signal is also added to the product signal developed in the output circuit of the balanced modulator 4i. From the preceding explanations in connection with the curves of Fig. 5, it will be clear that any appropriate amount of the rst -derivative signal `may beadded to thesignal output ofithe unit 4| further to reduce the geometricdistortion of the type .under considera- -tion appearing in the image produced by the `image-reproducing device associated with termina-ls'3l and 32.

Description of Fig.`7 apparatus Referring now to Fig. 7 of the drawings, there is represented a further embodiment of a respouse-modifying apparatus, portions of which yare-identical to those represented in Fig. 1. Ac-

cordingly, the same units in these two figures are designated by the same reference numerals. The response-modifying apparatus of Fig. '7 also includes a differentiating circuit 'I0 which is coupled between the output circuit of the amplifier 28 and the terminal 3| through a switch 13 .having contacts 'l2 and 14. An asymmetrical clipper 'H such as a diode limiter is coupled between the output circuit of unit i8 and contact 12 of the aforesaid switch.

Operation of Fig. 7 apparatus Before considering the general operation of the response-modifying apparatus of Fig. 7, it will be recalled that the brightness overshoots of the type represented by curve F of Fig. 3 for the Fig. 1 apparatus are due to the low velocity of the scanning spot of the electron beam of the imagereproducing device during a portion of the transient intervals, such as in the vicinity of times tb and te. The purpose of the units 'EG and/or 'Il of the Fig. 7 apparatus is to compensate for the effect of the low velocity of the scanning spot which produces brightness overshoots of the type 'just mentioned during a portion of the transient interval. Briefly considered, this brightness compensation is accomplished by developing the derivative of the control eiect applied to the with the curves of Fig. 8 of the drawings. Curve D' represents the variation with time of the scanning velocity of the electron beam of the image-reproducing device coupled to the terminals 3! and 32 by the action of the units 21 and 28 of the Fig. 7 apparatus. It will be seen that curve D of Fig. 8 is the same as curve D of Fig. 2. Curve I of Fig. 8 represents the second derivative of the unidirectional transient produced inthe well-known manner by the diiferentiating circuit 1B from the output signal of the units 21 and 28. With the switch I3 connected to contact i4, as represented in Fig. 7, the second derivative signal represented by curve I of Fig. 8 is applied by the diierentiating circuit 'I through the switch 3 and terminal 3l to the brilliancy-control electrode of the image-reproducing device of the image-reproducing system. Since brightness overshoot occurs during the second portion ti-tz of the transient interval to-tz and the first portion ifs-t4 of the next transient interval ts-ts, it will be seen from curve I that a negative control potential is applied to the brilliancy-conti'ol electrode of the image-reproducing device during the intervals -tl--tz and ts-ti.

This reduces the-*intensityv of thev electron ybeam during each interval when :a brightnessovershoot would normally. occur and thus vtends `par-- tially to compensate for the reductionsof the scanning velocity of the electronl beam.

When the switch 53 is connected to theterminal l2, a control signal having the waveform of curve J of Fig. 8 is applied to the terminal 3| associated with the brilliancy-con'trol 'electrode of the image-reproducing device. `It rwill be noted that the asymmetrical clipper 'il removes only the positive amplitude portions'offits input signal represented by curve I andproduces in its output circuit a control signal having only negative polarity pulses during the intervals to-tz and t3-t5. Thus the intensity of the .electron beam is modied in the manner mentioned above and the desired brightness compensation is effected. Since the signal of curve J doesnot include positive amplitude portions, the reproduction afforded by the image-reproducing. device in the region of blackduring the first portion of the interval of a positive-going transient and the second portion of the interval of a negativegoing transient is not impaired.

From the foregoing descriptions. of the various embodiments of the invention, it will be apparent that the response-modifying apparatus in accordance with the present invention for `use yin an image-reproducing system including van image-reproducing device represents a` simple and inexpensive apparatus. for .modifying .the apparent response of the image-reproducing device to unidirectional transients of. an-applied electrical signal. It will also .be clear thata response-modifying apparatus embodying L the present invention may be utilized ina television receiver to enable the latter to produce avery sharp picture. A television receiver employing a response-modifying apparatus in accordance with the invention is eiective to reduce "eye fatigue and other unpleasant reactions ordinarily experienced by some observerstof` television pictures. It will also be clear that a responsemodifying apparatus embodying the present invention is particularly suited for -use in .a television receiver 'having a cathode-ray tube with a relatively large display area, since thel apparatus is effective to improve the sharpness lof reproduction of the lpicture produced by the receiver.

While there have been described what'are': at present considered to be the preferred embodiments of this invention, it-'will be obvious to those skilled in the art that various changes 'and modifications may be made therein without departing from the invention.' and it is,V therefore, aimed in the appended claims to cover all such changes and modications vas fall-within the true spirit and scope of the invention.

What is claimed is:

1. An image-reproducing system comprising: an image-reproducing device for reproducing an image representing an electrical signal which may include unidirectional transients; a circuit for applying said signal to said device; means for normally scanning said device at a rpredetermined velocity; control circuit means coupled to said circuit and including .means for .deriving signals representative of the first derivative and of at least a higher order derivative :of' Said transients and including a .modulator device: responsive to said derived signals and effective. to developl a control. effectV representative jointly of said first derivative and .said .higher `order derivative of said transients of an applied signal; and electrical scanning means coupled to said control circuit means for utilizing said control effect to vary the velocity of scanning from said predetermined velocity successively in opposite senses'within a short interval after the initiation of a transient, whereby said device is conditioned to produce an image corresponding to said applied signal with modified transients.

2. In an image-reproducing system includingl an image-reproducing device for reproducing an image representing an electrical signal which may include unidirectional transients, an apparatus for modifying the apparent response-of the image-reproducing device to such transients comprising: a circuit for applying said electrical signal to an image-reproducing device; means for normally scanning an image-reproducing device at a predetermined velocity; control circuit means coupled to said circuit and including means for deriving signals representative of the first derivative and of at least a higher order derivative of said transients and including a modulator device responsive to said derived signals for developing a control signal representative jointly of said `first derivative and said higher order derivative of said transients; an electrical means coupled to said control circuit means for utilizing said control signal to vary the velocity of scanning from said predetermined velocity succces- Sively in opposite senses within a short interval after the initiation of a transient, whereby said device is conditioned to produce an image corresponding to said applied signal with modined transients.

3. In an image-reproducing system including an image-reproducing device for reproducing an image representing an electrical signal which may include unidirectional transients, an apparatus for modifying the apparent response of the reproducing device to such transients comprising: a circuit for applying said electrical signal to an image-reproducing device; means for normally scanning an image-reproducing device at a predetermined velocity; control circuit means coupled to said circuit and including means for deriving signals representative of the first derivative and of at least a higher order derivative of said transients and including a modulator device responsive to said derived signals for developing a control signal representative of the product of said first derivative and said higher order derivative of said transients; and electrical means coupled to said control circuit means for utilizing said control signal to vary the velocity of scanning from said predetermined velocity successively in opposite senses within a short interval after the initiation of a transient, whereby said device is conditioned to produce an image corresponding to said applied signal with modiiied transients.

1i. In an image-reproducing system including an image-reproducing device for reproducing an image representing an electrical signal which may include unidirectional transients, an apparatus for modifying the apparent response of the reproducing device to such transients coinprising: a circuit for applying said electrical signal to an image-reproducing device; means for normally scanning van image-reproducing device at a predetermined velocity; differentiating means coupled to said circuit for developing a first signal representative of the rst derivative of said transients and a second signal representative of the second derivative of said transients;

means for modulating said second representative signal with said rst representative signal to produce a control effect which eiectively is the product of said derivatives; and electrical means coupled to said modulator means :for utilizing said control effect to Vary the velocity of scanning from said predetermined velocity successively in opposite senses within a short interval after the initiation of a transient, whereby said device is conditioned to produce an image corresponding to said applied signal with modified transients.

5. In an image-reproducing system including an image-reproducing device for reproducing an image representing an electrical signal which may include unidirectional transients, an apparatus for modifying the apparent response of the reproducing device to such transients comprising: a circuit for applying said electrical signal to an image-reproducing device; means for normally scanning an image-reproducing device at a predetermined velocity; differentiating means for developing a first signal represntative of the first derivative of said transients and a second signal representative of the second derivative of said transients; a balanced modulator for modulating said second representative signal with said first representative signal to produce a control effect which is eiTectively the product oi` said representative signals; and electrical means coupled to said balanced modulator for utilizing said control effect to vary the velocity of scanning from said predetermined velocity successively in opposite senses Within a shori-I interval after the initiation of a transient, whereby said device is conditioned to produce an image corresponding to said applied signal with modified transients.

6. In an image-reproducing system including an image-reproducing device for reproducing an image representing an electrical signal which may include unidirectional transients, an apparatus for modifying the apparent response of the reproducing device to such transients comprising: a circuit for applying said electrical signal to an image-reproducing device; means for normally scanning an image-reproducing device at a predetermined velocity; a first differentiating means for developing a first signal representative of the rst derivative of said transient; a second differentiating means for developing a second signal representative of the second derivative of said transient; means having a nonlinear translating characteristic coupled to said rst differentiating means for symmetrically clipping said first representative signal; a balanced modulator coupled to said second differentiating means and said clipping means to produce a control signal which is effectively the product of said second representative signal and the output signal of said clipping means; and electrical means coupled to said balanced modulator for utilizing said control effect to vary the velocity of scanning from said predetermined velocity successively in opposite senses within a short interval aitc-r the initiation of a transient, whereby said device is conditioned to produce an image corresponding to said applied signal with modied transients.

'1. In an image-reproducing system including an image-reproducing device for reproducing an image representing an electrical signal which may include unidirectional transients, an apparatus for modifying the apparent response of the reproducing device to such transients comprising: a circuit for applying said electrical signal to an image-reproducing device; means for normally scanning an image-reproducing device at a predetermined velocity; differentiating means for developing a first signal representative of the rst derivative of said transients and a second signal representative of the second derivative of said transients; means for modulating said second representative signal with said' first representative signal to produce a control effect which is the product of said representative signals; means for effectively adding said rst representative signal to said control eiect to produce a second control effect; and electrical means coupled to said modulating means and said adding means for utilizing said second control eiect to vary the velocity of scanning from said predetermined velocity successively in opposite senses Within a short interval after the initiation of a transient, whereby said device is conditioned to produce an image corresponding to said applied signal with niodied transients` 8. In an image-reproducing system for a television receiver including an image-reproducing device having a hriiliancy-control electrode for reproducing an image representing a television signal which may include unidirectional transients, an apparatus for modifying the apparent response of the reproducing device to such transients comprising: a circuit including a videofrequency amplifier for applying said television signal to the brilliancy-control electrode of an image-reproducing device, said amplifier having a restricted pass band such that it is ineffective to apply to said electrode the high-frequency components of said television signal; means for normally scanning an image-reproducing device at a predetermined velocity; control circuit means coupled to said circuit and including means for deriving signals representative of the rst derivative and of at least a higher order derivative of said transients and including a modulator device responsive to said derived signals and effective to develop a control effect representative of the product of said first derivative and said higher order derivative of said transients of an applied signal; and electrical means coupled to said control circuit means for utilizing said control effect to vary the velocity of scanning from said predetermined velocity successively in opposite senses within a short interval after the initiation of a transient, whereby said high-frequency components are effectively applied to said device and said device is conditioned to produce an image corresponding to said applied signal with modified transients.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,134,094 Andrieu Oct. 25, 1938 2,182,326 Urtel Dec. 5, 1939 2,227,630 Carnahan Jan. 7, 1941 2,243,599 HerbstJ May 27, 1941 FOREIGN PATENTS Number Countryl Date 413,401 Great Britain July 19, 1934 483,935 Great Britain Apr. 28, 1938 

